The present disclosure relates to radiation detection devices, and more particularly, the present disclosure relates to radiation dosimeters.
A radiation detector is any device that generates a measurable signal in response to exposure to ionizing radiation. A radiation dosimeter is a radiation detector whose output is the amount of energy deposited per unit mass of tissue when tissue is exposed to the same field of ionizing radiation. The two terms are generally not interchangeable, because the physical processes of radiation interaction and transport can differ significantly between the medium of the detector and tissue. The translation of a measured signal from a radiation detector into a dose (energy absorbed per unit mass) typically requires knowledge of the type and energy of the radiation incident on the detector. Appropriate calibration factors can then be employed to arrive at the desired result: absorbed dose.
An ideal dosimeter would have the following characteristics: the ability to absorb radiation in a manner that closely mimics absorption by the human body, independent of the type of radiation, its energy, direction or dose rate, and the ability to provide accurate, reproducible readings in real time. Currently, no radiation dosimeter in routine clinical use can accomplish these goals simultaneously. Widely used dosimeters such as ionization chambers, diodes, thermoluminescent dosimeters (TLDs), metal oxide semiconductor field-effect transistors (MOSFETs) or radiochromic film all fail in at least one of these requirements.
For example, ionization chambers have relatively small energy dependence at photon energies in the range of 1-10 MeV, however, in the diagnostic imaging energy range of photon energies in the range of 20-140 keV, the dependence of the signal on photon energy becomes much more substantial.
In FIGS. 1A and 1B, the attenuation coefficient of photons as a function of energy is shown for various materials. At energies below approximately 150 keV, the attenuation coefficient of silicon diverges substantially from that of tissue (by up to a factor of nearly 7). In this energy range the attenuation coefficient begins to depend very strongly on the elemental composition of the material in through which the photons pass.
Diodes are composed primarily of silicon, which introduces a substantial photon energy dependence at lower photon energies due to the significantly enhanced photoelectric cross section relative to that of tissue, as can be seen from FIGS. 1A and 1B. The same is true of MOSFETs. Tissue-equivalent dosimeters such as radiochromic film and lithium fluoride thermoluminescent dosimeters (TLDs) are not capable of generating real-time signals (radiochromic film requires a development period of at least 12 hours and TLDs are stored for at least 4 hours prior to readout). They also require access to additional infrastructure (e.g. a scanner for film and a specialized reader for TLDs) that is not portable and prohibits readout at the time and location of exposure.